An exemplary prior art telephone system is shown in FIG. 1. The telephone system 100 includes a V.PCM digital modem 110 which is connected to a digital channel 112 that terminates at central office 114. The V.PCM digital modem, also referred to as a V.90 modem, is described in the International Telecommunications Union, Telecommunication Standardization Sector (ITU-T), Recommendation V.90 (1998), herein incorporated by reference in it's entirety. The V.PCM digital modem 110 transmits PCM octets through the digital channel 112 to the central office 114. At the central office 114 the bytes are processed by a convert-and-filter device 116 which includes a μ-law (or A-law) digital-to-analog converter (“DAC”) followed by an analog PCM filter. The output of the convert-and-filter device 116 is then transmitted to a subscriber via an analog channel 118 which consists, for example, of two copper wires which typically are referred to as a twisted pair.
In some telephone systems the analog channel 118 consists of multiple ones of these twisted pairs, each twisted pair referred to as a “line”, which are then in turn connected to a unit (“EU”) 120 which is located outside the central office 114. The unit 120 includes a conversion device 122 which includes an analog front end (“AFE”) and a μ law (or A-law ) analog-to-digital converter (“ADC”). In the conversion device 122, once each line is processed by the μ law (or A-law) ADC, an output bit stream of distorted PCM octets 124 is sent to a second unit 126 (called “RU”) via a digital modem (e.g., a digital subscriber line (“DSL”) modem), not shown. At the RU unit 126 the bits are processed by conversion device 128 which includes another μ-law (or A-law) DAC and a PCM filter. The resultant analog signal is sent to a subscriber via analog channel 130 and a V.PCM analog modem 132.
For many years the public digital telephone network (DTN) has been used for data transmission between modems. Typically, a modulated carrier is sent over a local loop to a service provider (e.g., a Regional Bell Operating Company), whereupon the service provider quantizes the signal for transmission through the DTN. A service provider that is located near the receiving location converts the digital signal back to an analog signal for transmission over a local loop to the receiving modem. This system is limited in the maximum achievable data rate at least in part by the sampling rate of the quantizers, which is typically 8 kHz (which rate is also the corresponding channel transmission rate, or clock rate, of the DTN).
Furthermore, the analog-to-digital (A/D) and digital-to-analog (D/A) conversions are typically performed in accordance with a non-linear quantizing rule. In North America, this conversion rule is known as mu.-law. A similar non-linear sampling technique known as A-law is used in certain areas of the world such as Europe. The non-linear A/D and D/A conversion is generally performed by a coder/decoder (“codec”) device located at the interfaces between the DTN and local loops. Alternatively, these devices are referred to herein as a DAC and an ADC.
It has been recognized that a data distribution system using the public telephone network can overcome certain aspects of the aforesaid limitations by providing a digital data source connected directly to the DTN, without an intervening codec. In such a system, the telephone network routes digital signals from the data source to a client's local subscriber loop without any intermediary analog facilities, such that the only analog portion of the link from the data source to the client is the client's local loop (plus the associated analog electronics at both ends of the loop). The only codec in the transmission path is the one at the DTN end of the client's subscriber loop.
Because of the existence of two DACs in the exemplary telephone system 100, those lines effectively cannot carry 56K signals. Thus, subscribers who use 56K modems are generally unable to use these lines.